F40 2002 Yamaha - RPMs Wandering

[jbinks]

I own a 2002 F40 electric start Yamaha - 3 cylinder carbed engine. the motor is on an 18 ft pontoon boat with 23" pontoons. It runs great except for one annoying trait. When set to cruise at 4500 rpm the rpms wander slightly - enough so that you can both hear and feel the difference. It seems by the tach that variance is about 100 to 200 rpm. Everything else about the motor is fine. It starts well, acceleration from start to wot is smooth and it idles well. WOT rpm is 5600. The only problem is the wandering rpms when cruising around 4500 rpm. It is annoying and drives me crazy. Does anyone else have this problem and if you have a suggestion please let me know.

 

[andymach23]

Re: F40 2002 Yamaha - RPMs Wandering

I had this engine on my old boat. It did exactly the same thing. The revs would fluctuate by say 100 RPM, just momentarily, then return to the throttle selected revs. I posted on here about it. I did not get to the bottom of it and ended up putting it down to the currents in the water changing. My normal cruising area is very tidal. Either I forgot about it or it just went away. Last year was my last sesaon with the motor and I can't remember it doing it.

I had a run last year on a river at about 4.3 to 4.5K revs for an hour and the motor ran sweet with no fluctuation.

The fluctuation did not spoil my enjoyment of this engine. I loved it. It was very quiet and very good on fuel.

I now have an F80 on my new boat and it makes oil, doesn't pee very well on the flush attachment and is a much less pleasing engine due to the additional noise. 'Horses for courses' I suppose with a larger engine.

Cheers

Andy

 

[jbinks]

Re: F40 2002 Yamaha - RPMs Wandering

Thanks Andy,
I guess I may just have to put up with it. Besides that one problem the motor is great.

 

[Yamajoe]

Re: F40 2002 Yamaha - RPMs Wandering

Currents are a factor; does the RPM wander at WOT?

 

[jbinks]

Re: F40 2002 Yamaha - RPMs Wandering

No the RPMs are constant except in the range between 4,00 and 5,000. That is the only place it happens. Any ideas?

 

[Yamajoe]

Re: F40 2002 Yamaha - RPMs Wandering

If this carbed, Clean the carbs your problem is likely clogged bypass holes which feed fuel into venturi at mid range. use mild compressed air to blo into thos holes and clean all jets. Your main jet seems to ok if you are not experiencing the miss at WOT, but clean these too. You need to sincronise the carbs, only way to do this properly is with a vacuum gauge or manometer.

 

[jbinks]

Re: F40 2002 Yamaha - RPMs Wandering

Thanks Yamajoe,
I will try this next week as I will be on holidays and will have the time to do it.
John